A known mixer device is for example described in British patent specifications 930.339 and 1.475.216 and the European patent application 48.590.
With such a known mixer device of the so-called cavity transfer type the mixing means consist of mutually coacting mixing cavities arranged in the exterior surface of the rotor and in the interior surface of the stator. These mixing cavities can be arranged peripherally in staggered rows in the rotor and stator as in the European patent application 48.590 whereby rows of mixing cavities are arranged axially staggered in the stator and rotor.
The known mixer device according to the above stated patent publications requires the arrangement of mixing cavities in the interior surface of the stator across an axial length which for an extruder is substantially equal to the mixing section of the rotor and in the case of an injection moulding machine substantially equal to the stroke length of the mixing section of the rotor. The arrangement of these mixing cavities in the interior surface of the stator entails relatively high production costs, while the mixing sections in the rotor and stator must be geared to one another, which decreases the extent of interchangeability.
Another similar mixer device is the pin mixer whereby the mixing means comprise pins arranged spread in axial planes over the interior surface of the stator which extend as far as breaks in the rotor screw thread(s).
The German patent application 2,327,540 discloses a mixer device attached to the downstream end of an extruder screw. This mixer device comprises a rotor and a mixing ring mounted for rotation on and around the rotor. The mixing ring consists of several sleeves spaced apart by dam rings and separating an outer annular mixing chamber from an inner annular mixing chamber. The outer mixing chamber faces the inner surface of the stator, whereas the inner chamber faces the outer surface of the rotor. Via bores the inner and outer mixing chamber communicate with downstream mixing chambers. The distributive mixing action of this mixing ring is relatively low, and the main object of the mixing ring is a radial change in place relative to the rotor and stator. Warm and cold material change place during the downstream movement and accordingly the thermal homogenity of the mixed material is improved. Furthermore, the residence time of the material to be mixed does not affect or to a very minor extent affects the mixing quality.